Identification of a Micropeptide on Lipid Droplet That Mediates Lipid Storage and Insulin Sensitivity

Great progress in sequencing technologies and computational biology suggests that the noncoding sequence in mammalian genome is rich in functional elements to produce proteins. Many RNA molecules, mis-annotated as noncoding, actually harbor small open reading frames that are predicted to code proteins called micropeptides. Some micropeptides have been discovered and verified to play critical roles in multiple biological processes. Lipid droplet (LD) is a unique cellular organelle, conserved from bacteria to humans, and closely associated with cellular lipid metabolism and metabolic disorders. However, no micropeptide has been identified on LDs. Here we for the first time explore this possibility. Proteins with small molecular weight from isolated LDs of myoblasts were enriched by electrophoresis and subjected to proteomic analysis, and LD-associated micropeptides (LDAMP) were identified using mass spectrometry coupled with a home-made protein database. A total of 15 micropeptides were identified. One of them was studied further and termed LDAMP1. Similar to LD resident proteins, LDAMP1 was degraded by proteasome. LDAMP1 was localized on LDs by confocal imaging, cell fractionation, as well as immunogold labeling. Using CRISPR/Cas9-mediated genome editing technique, endogenous expression of LDAMP1 was validated by FLAG tag knock-in. Moreover, oleate-induced triacylglycerol accumulation was significantly decreased when myoblasts expressing LDAMP1 and, in turn, the rescue of palmitate-inhibited insulin sensitivity was obviously reduced. Taken together, we report for the first time that translatable noncoding RNA-derived peptides were identified on LDs and a novel LD protein LDAMP1was discovered, which paves a new way for exploring LD dynamics and related metabolic functions.